1. Field of the Invention
The present invention relates generally to a cathode ray tube and more particularly to a cathode ray tube having a thin film with light selective absorptivity on the front surface of its faceplate.
2. Description of the Related Art
A cathode ray tube functions to display characters, images, etc. In this case, an electron beam emitted from an electron gun provided in a neck portion of an envelope impinges upon a phosphor screen in which dot-shaped or stripe-shaped red-, green- and blue-emitting phosphors are provided regularly on the inner surface of a glass faceplate. In general, a glass plate (neutral filter) having a substantially constant transmissivity with respect to visible radiation is attached to the front surface of the faceplate, thereby to suppress reflection of external light on the phosphor screen formed on the inner surface of the faceplate and to improve the contrast of displayed images.
The external light reflected by the phosphor screen formed on the inner surface of the faceplate is attenuated in proportion to T.sup.2 by virtue of the neutral filter (glass plate), and thereby the contrast of displayed images is improved. However, supposing that the transmissivity of the glass plate is T, the brightness of images decreases in proportion to the transmissivity T of the glass plate. This technique, therefore, is not desirable.
To address this problem, there has been proposed a cathode ray tube having on the front surface of its faceplate a glass plate containing neodymium oxide (Nd.sub.2 O.sub.3) and having light selective absorptivity, thereby suppressing a decrease in brightness of images and improving the contrast of images (Published Unexamined Japanese Patent Application (PUJPA) No. 57-134848, PUJPA No. 57-134849, and PUJPA No. 57-134850). By virtue of the inherent absorption properties of neodymium oxide, the glass plate has a main absorption band in a range of 560 nm to 615 nm and a sub absorption band in a range of 490 nm to 545 nm. Thus, the color purity of red and blue images can be increased.
This type of glass plate, however, fails to improve the contrast of images remarkably, even though it has the above-mentioned light selective absorptivity. There is known a method of evaluating the improvement of the contrast, on the basis of BCP (Brightness Contrast Performance). The BCP is given by BCP=.DELTA.B/.sqroot..DELTA.Rf, where .DELTA.B is the decrease in brightness, .DELTA.Rf is the decrease in external light reflectance. The BCP represents a contrast improvement ratio evaluated on the basis of a system using a neutral filter. When the characteristic of a filter having the selective absorptivity of neodymium oxide is evaluated on the basis of the BCP, it is found that the BCP is 1.ltoreq.BCP.ltoreq.1.05 and the contrast is not improved sufficiently.
The glass plate containing neodymium oxide has the main absorption band in a wavelength range of 560 nm to 615 nm. In addition, a steep portion of a peak width of 5 nm to 10 nm is included within the region of 560 to 570 nm of the main absorption band. Thus, the color (body color) of the glass plate itself is changed by external light. In particular, the body color of the glass plate becomes reddish under the light of an incandescent lamp. As a result, low-luminance parts of an image, such as a black part and a shadow part, become reddish, and it becomes difficult to see the image. In addition, the quality of image is degraded. Furthermore, since neodymium is an expensive material, the manufacturing cost of the glass plate increases.